To cope with the rapid growth of traffic demands for data services, on the basis of a cellular network of a conventional macro base station, a large number of femto base stations, for example, Small cell small cell base stations and femto cell base stations are densely deployed in some indoor or outdoor hot spots. The femto base station is characterized by a small coverage area and low transmit power, is suitable for providing high-rate data transmission services, and can offload data traffic of a macro cellular network, thereby reducing a network deployment cost of a carrier.
However, to ensure normal data transmission between a macro base station and femto base stations, clock synchronization needs to be implemented between the femto base stations and between the femto base stations and the macro base station.
To identify synchronization states of the femto base stations, a synchronization level of each femto base station is predefined. FIG. 1 is a schematic structural diagram of a multi-hop synchronization system.
As can be seen from FIG. 1, a macro base station acquires timing synchronization information from a global positioning system (GPS), that is, a synchronization level is set to 0; femto base station 1 obtains timing synchronization information from the macro base station, that is, a synchronization level is set to 1; femto base station 2 obtains timing synchronization information from femto base station 1, that is, a synchronization level is set to 2; . . . ; and femto base station M obtains timing synchronization information from femto base station M−1, that is, a synchronization level is set to M.
A femto base station that acquires synchronization information from another base station is referred to as a synchronization base station, and a base station that provides another base station with synchronization information is referred to as a synchronization source base station. For example, the macro base station provides femto base station 1 with synchronization information, the macro base station is a synchronization source base station for femto base station 1, and femto base station 1 is a synchronization base station for the macro base station. Femto base station 1 provides femto base station 2 with synchronization information, femto base station 1 is a synchronization source base station for femto base station 2, and femto base station 2 is a synchronization base station for femto base station 1. It can therefore be seen that in a wireless communications system, any base station may be a synchronization base station, or may be a synchronization source base station.
Using the multi-hop synchronization system mode for synchronization between base stations in a communications network system is subject to the following defect: When more synchronization levels are involved in synchronization information, the synchronization error also increases gradually; therefore, a relatively large synchronization error is caused between a base station having a high synchronization level and a base station having a low synchronization level, resulting in relatively low accuracy in synchronization between base stations in a communications system.
In consideration of factors such as deployment scenario, cost, and implementation complexity, in a group of femto base stations that are deployed densely, synchronization information may be acquired in manners such as network listening and blind detection, so as to implement clock synchronization. That is, a synchronization base station tracks a synchronization source base station by means of network listening, and acquires synchronization information of the synchronization source base station in a blind detection manner, so as to implement clock synchronization between the synchronization base station and the synchronization source base station.
For example, first, a synchronization base station measures, by means of network listening, a common reference signal (CRS) of a synchronization source base station that carries clock synchronization information; next, the synchronization base station determines, according to the common reference signal obtained by listening, in a resource (including multiple subframes) used for sending the common reference signal, a subframe that carries a CRS sequence and a subframe that does not carry a CRS sequence; and furthermore, the synchronization base station further detects a position of a carrier resource of the subframe that does not carry a CRS sequence, and determines a synchronization level of the synchronization source base station according to the detected position of the carrier resource.
For example, a synchronization source base station sends a common reference signal, and a subframe that does not carry a CRS sequence is carried at position 1 of a carrier resource for sending the common reference signal, or it may also be defined that a synchronization level of the synchronization source base station is 1; and when a synchronization base station detects, according to the received common reference signal of the synchronization source base station, that the position of the carrier resource of the subframe that does not carry a CRS sequence is 1, it is determined that a synchronization level of a synchronization source base station that receives the common reference signal is 1.
When a synchronization level of a synchronization source base station is acquired in the above manner, the following defect exists:
On some synchronization source base stations, a multimedia broadcast multicast service (MBMS) is configured, but a CRS signal is not configured. When a synchronization base station still detects a position of a carrier resource of a subframe that does not carry a CRS sequence to determine a synchronization level of the synchronization source base station, because no CRS signal is configured on the synchronization source base station, a position that carries a carrier resource for sending MBMS is liable to be determined as a position of a carrier resource of a subframe that does not carry a CRS sequence, and the synchronization level of the synchronization source base station is determined according to the position of the carrier resource configured to send MBMS. For example, a synchronization level of a synchronization source base station is 1, but position 2 is configured as a position of a carrier resource for sending MBMS, so that a synchronization base station determines, according to the detected position 2 of the carrier resource for sending MBMS, that the synchronization level of the synchronization source base station is 2. This will result in occurrence of an error in determining the synchronization level, which directly affects synchronization information acquired by the synchronization base station, and reduces accuracy that the synchronization base station performs synchronization tracking.